One would conclude from the above measurement and LL best-fit, Fig. REF ,
that the sample contains only Fe\(^{3+}\)  with a concentration
\(n\) (Fe)=1.539 \(\times \)  10\(^{18}\)  g\(^{-1}\)  [some 158 ppm] and
with a Larmor susceptibility \(\chi _L\) =5.030 \(\times \)  10\(^{-7}\)  emu/gOe (or
cm\(^3\) /g). Though the above is one of our worst-case examples, the agreement
between our data and expression (REF ) is generally unsatisfactory for all studied
samples, indicating that an extra contribution is missing. We also notice that:
1) The data agreement with the LL form, Eq. (REF ), is generally worst at the
lowest temperatures and fixed \(H\) , even for the Fe\(^{3+}\) -only scenario and
will not improve by allowing other contaminants having \(J_s<\) 5/2 in the best-fit.
Imposing the Fe\(^{2+}\) -only scenario (thus yielding 2.027 \(\times \)  10\(^{18}\) 
g\(^{-1}\)  [some 208 ppm] of Fe) makes the disagreement much worse;
2) This naively SQUID-extracted Fe-concentration is somewhat in disagreement with
the result from our own very precise mass-spectrometry (Inductively-Coupled Plasma
Mass Spectrometry, ICP-MS, hereafter referred simpy as MS) analysis of the sample
giving \(n\) (Fe)=(1.469 \(\pm \)  0.006) \(\times \)  10\(^{18}\)  g\(^{-1}\)  only
[meaning 151.0 \(\pm \)  0.6 ppm];
moreover, a quick textbook-formula [1]} estimate of Larmor
susceptibility yields for Duran: \(\chi _L \simeq \)  4.03 \(\times \)  10\(^{-7}\)  emu/gOe;
3) It is very surprising that Fe\(^{3+}\)  should be considered predominant in the
Duran shard, because of the faint greenish tinge of the specimen (Figs. 1(a–c) in the
SI) like in most ordinary clear silicate glass items which indicates rather Fe\(^{2+}\) 
predominance [2]}.
